In the past various extension ladders comprising separable sections have been provided. Typical are the ladders disclosed in U.S. Pat. No. 1,555,344 to Whitney issued Sept. 29, 1925; Australian Pat. No. 289,370 to Paynter of Mar. 4, 1969 and French Pat. No. 1,252,593 to Roth issued Dec. 19, 1960. In each of these patents the ladders comprise separable base and extension sections which are coupled together at longitudinally spaced locations by means of co-operating rung and slot couplings whereby rungs and extensions of rungs are received in longitudinally extending slots.
These prior art extension ladders all suffer a major disadvantage that to couple the two separable sections of the ladder together requires the two sections to be precisely positioned with respect to each other in longitudinal alignment so that the slot and rung coupling means at the two spaced locations may be concurrently engaged. Suitable positioning of the ladder sections may be extremely difficult particularly by a single person when the ladder sections are heavy or when no suitably flat terrain is available on which to lay both of the ladder sections. When used outdoors, vegetation, mud and snow may catch on the sections or partially block the slots increasing the difficulty of coupling the sections together. Further in frigid wintery conditions protective clothing such as mitts and gloves impair the ability of an individual to precisely locate the ladder sections as is required for coupling.
Servicemen working for utility companies such as hydro electric suppliers, cable television suppliers and telephone companies require ladders for day-to-day use. For example in Canada, telephone company servicemen require almost daily a ladder which can extend to 17 feet. Less frequently they will require a ladder which extends to 21 feet. In the past these servicemen have been provided with a two piece telescoping extension ladder which is 12 feet in length when unextended and may extend to 21 feet. Recently, however, telephone servicemen have been provided with several automobiles of smaller size than previously, with the result that the existing 12 foot ladders are longer than the new automobiles and present a safety hazzard. To overcome this problem new ladders were proposed comprising a telescoping two piece ladder of 10 foot length unextended which may extend to 17 feet. To provide the ladder with the capability to extend 21 feet when necessary a third detachable section has been proposed. The disadvantages of the prior art extension ladders have made the use of known separable extensions unsatisfactory particularly in regard to difficulties in coupling the separable section to the rest of the ladder by a single serviceman in the field. Another disadvantage of the known separable extension ladder is the absence of any provision for positive locking of the separable extension to the rest of the ladder as is required to ensure safe use of the ladder.